1. Field of the Invention
The present invention generally relates to precursor source reagent metal-organic compositions, a chemical vapor deposition (CVD) process utilizing such precursor source reagent metal-organic compositions for formation of metal films on substrates, and to CVD processes utilizing liquid delivery and volatilization of such precursor source reagent metal-organic compositions for supplying a source reagent vapor to a CVD reactor.
2. Description of the Related Art
Chemical vapor deposition is widely used for the formation of metal films on a variety of substrates. CVD is a particularly attractive method for forming metal films because it is readily scaled up to production runs and because the electronics industry has a wide experience and an established equipment base in the use of CVD technology which can be applied to CVD processes.
CVD requires source reagents that are sufficiently volatile to permit their gas phase transport into the decomposition reactor. The source reagent must decompose in the CVD reactor to deposit only the desired element(s) at the desired growth temperature on the substrate. Premature gas phase reactions are desirably avoided, and it generally is desired to controllably deliver source reagents into the CVD reactor to effect correspondingly close control of stoichiometry.
Many potentially useful metals do not form compounds which are well suited for CVD. Although some source reagents are solids which are amenable to sublimation for gas-phase transport into the CVD reactor, the sublimation temperature may be very close to decomposition temperature. Accordingly, the reagent may begin to decompose in the lines leading to the CVD reactor, and it then becomes difficult to control the stoichiometry of the deposited films.
Accordingly, there is a continuing search in the art for improved source reagent compositions which are amenable to vaporization to form the source component vapor for CVD processes.
U.S. Pat. No. 5,204,314 discloses a process for supplying an involatile source reagent in vapor form for CVD, in which reagent source liquid is flash vaporized on a high surface-to-volume ratio structure, following which the vaporized reagent is flowed to the CVD reactor, for deposition of the desired metal or other component on the target substrate in the reactor.
In the chemical vapor deposition of multicomponent material systems, multiple source reagents are delivered to the CVD reactor. A particularly advantageous way of delivering multiple source reagents is to accurately mix neat liquid source reagents or liquid solutions of source reagents and then flash vaporize the mixture and deliver the resulting vapor to the reactor. It is possible in this situation for the reagents to undergo reactions, either in the liquid phase before vaporization or in the gas phase after vaporization. If these reactions convert a source reagent to an insoluble or non-volatile product, or to a material of different chemical or physical properties, then the elements contained in that product will not reach the substrate and the stoichiometry of the deposited film will be incorrect.
Examples of this problem (wherein Et is ethyl; tBu is tert-butyl; iPr is isopropyl; and thd is tetramethylheptanedionate) include the following:
(i) during deposition of PbZrXTi1−XO3, using (Et)4Pb, Zr(OtBu)4, and Ti(OiPr)4 source reagents, ligand exchange between the Zr and Ti reagents resulted in formation of Zr(OiPr)4 (and perhaps other products of which Zr(OiPr)4 is a monomer), which had very low volatility and which condensed in the gas manifold or vaporizer;
(ii) when solutions of Ba(thd)2 and Ti(OiPr)4 were mixed prior to vaporization, an insoluble precipitate was formed, presumably Ba(OiPr)2; and
(iii) when solutions of Pb(thd)2 and Ti(OiPr)4 were mixed in butyl acetate, the reagents reacted to form compounds of differing physical properties, such as Pb(OiPr)2 and Ti(OiPr)2(thd)2.
Another specific example illustrating this problem is the preparation of films of strontium bismuth tantalate and strontium bismuth niobate (SrBi2Ta2O9 and SrBi2Nb2O9) by CVD for use in non-volatile ferroelectric random access memories. The most commonly used strontium source reagents are β-diketonate complexes such as Sr(thd)2. When a solution is heated containing the following source reagents for deposition of SrBi2Ta2O9:
Sr(thd)2; Ta(OEt)5; and Bi(Ph)3 wherein Ph=phenyl,
the ethoxide ligands of the tantalum reagent exchange with the thd ligands of the strontium reagent, leading to the formation of undesirable strontium alkoxide species that have reduced volatility and that can decompose in the vaporization zone. Alternatively, when these reagents are provided separately in bubblers, similar ligand exchange reactions occur in the gas phase; the resulting solids constrict the gas lines or alter the film stoichiometry.
In certain instances, such problems can be avoided by using identical ligands on the metals to make ligand exchange a degenerate reaction (i.e., where the exchanging ligand is identical to the original ligand). Examples of this approach include the use of tetraethylorthosilicate, triethylborate and triethylphosphite for deposition of borophosphosilicate glasses (J. Electrochem. Soc., 1987, 134(2), 430). In many instances, however, this method for avoiding the problem is not possible because the appropriate compound does not exist, is too unstable or involatile to be used for CVD, or otherwise has disadvantageous physicochemical material properties. For example, for deposition of PbZrXTi1−XO3, a reagent system with identical ligands is problematic because while Pb(thd)2 and Zr(thd)4 are stable and volatile, Ti(thd)4 does not exist and Ti(thd)3 is extremely air sensitive. Similarly, while Ti(OtBu)4 and Zr(OtBu)4 are stable and volatile, Pb(OtBu)2 is thermally unstable at temperatures required for volatilization.
The foregoing problems are also encountered in the circumstance where the metal source reagent is provided in a liquid solution and the solvent contains moieties which react with ligands of the source reagent compound to produce undesirable ligand exchange reaction by-products which display different physical properties and are involatile or insoluble.
Accordingly, it is an object of the present invention to provide a CVD process utilizing improved metal source reagent compositions for the deposition of corresponding metals and metal oxides.
It is another object of the invention to provide a CVD process utilizing improved metal source reagent compositions in liquid or solution form, to simultaneously deliver the constituent metal(s) to a deposition locus such as a chemical vapor deposition chamber.
It is a further object of the present invention to provide a CVD process utilizing liquid compositions of such type which are resistant to deleterious ligand exchange reactions.
It is yet another object of the invention to provide a liquid delivery and chemical vapor deposition process in which precursor source reagent compositions are volatilized and the resulting vapor is transported to the CVD reactor for deposition of the desired component(s) on a substrate disposed in the reactor.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.